1. Field of the Invention
The present invention relates to a system that uses high-frequency signals to modulate optical signals, and particularly to a reciprocating optical modulation system that can generate an optical signal having a frequency that is a number of times higher than that of the applied electrical signal.
2. Description of the Prior Art
Optical modulation by high-frequency signals is generally accomplished by inputting an optical carrier wave and a high-frequency electrical signal to an optical modulator and performing intensity modulation or phase modulation or other such modulation. In the case of this method, in order to obtain a sideband having a frequency that is at least as high as that of the applied high-frequency electrical signal, the high-frequency electrical signal is multiplied to form an even higher-frequency signal that is used for the optical modulation. However, even when the high-frequency electrical signal is thus multiplied, the maximum modulation frequency is determined by the upper limit of the electrical signal. Thus, in terms of frequency, an electrical signal that is multiplied or amplified or other such signal is limited by the maximum characteristic of the electrical circuit. There is therefore a need to overcome this drawback.
There have been reports of attempts to produce a sideband with a frequency higher than that of the applied high-frequency signal, using phase modulation with a high modulation index. One reference (xe2x80x9cGeneration of Ultrashort Optical Pulses Using Domain-Inverted External Phase Modulator,xe2x80x9d by T. Kobayashi, OYO BUTURI, vol. 67 No. 9 (1998), pp. 1056-1060) describes applying a 16.26 GHz electrical signal to an optical modulator having a waveguide formed of an electro-optical crystal of LiTaO3 on which a stripline resonator was disposed. With a modulation index set at 87 radial, the spectral bandwidth was around 2.9 THz.
However, with the above type of phase-modulation configuration that uses a high modulation index, in order to increase the amplitude of the high-frequency electrical signal so as to obtain the high modulation index, a stripline resonator is used as the modulator electrodes, making it difficult to change the modulation frequency. Although it is easy to think of a configuration in which using a resonator as the modulator electrodes can be avoided by amplifying the electrical signal, thereby making it possible to readily change the modulation frequency, it is well known that the amplifier would place an upper limit on the high-frequency electrical signal.
In view of the above, an object of the present invention is to provide a reciprocating optical modulation system that can readily generate high-order sidebands even with a high-frequency electrical signal having a smaller amplitude than that of the prior art phase modulation configuration described above that uses high modulation-index setting.
To attain the above object, the present invention provides a reciprocating optical modulation system, the system comprising: means that, taking n as a predetermined integer of 1 or more, modulates light of a predetermined frequency to produce an nth order sideband group thereof; means that modulates the nth order sideband group to produce an (n+1)th order sideband group; and means that selects at least part of the (n+1)th order sideband group. Here, xe2x80x9c(n+1)th order sidebandxe2x80x9d refers to a sideband (n+1)-times the modulation frequency that is frequency-separated from the carrier wave, and the xe2x80x9c(n+1)th order sideband groupxe2x80x9d refers to two sidebands located symmetrically with respect to the carrier wave.
The reciprocating optical modulation system of the present invention also includes a light path that is shortened by being folded by a reflection means. The system also includes a plurality of modulation means, to which at least one of sideband groups of a different order is input, a configuration that can reduce the cost of the system by decreasing the number of modulators used.
The present invention also provides a reciprocating optical modulation system comprising first reflection means that prior to modulation passes light of the predetermined frequency and reflects light of other frequencies; and second reflection means that with respect to the predetermined integer n of 1 or more, passes (n+1)th sidebands and reflects other light. One example of the system configuration comprises a laser light source, the first reflection means comprised of a narrow-bandpass filter, an optical modulator, and the second reflection means comprised of a band-limiting filter. This reciprocating optical modulation system can further includes a laser light source and an optical modulator, wherein the first reflection means is comprised of a narrow-bandpass filter, and the second reflection means is comprised of a band-limiting filter. The reciprocating optical modulation system can further includes a phase modulator, wherein the first reflection means is comprised of a narrow-bandpass filter, and the second reflection means is comprised of a band-limiting filter.
The invention also provides a reciprocating optical modulation system that further includes means for converting part of the optical output into electrical signals, and means for re-inputting the electrical signals to the modulation signal of the phase modulator. In another example, the system also includes means that uses a nonlinear element for combining a portion of the light input to the system with sideband output from the system, and means for extracting from the mimed signals electrical signals of a predetermined frequency band.
Further features of the invention, its nature and various advantages will be more apparent from the accompanying drawings and following detailed description of the invention.